A toner which emits fluorescence in a visible light region by irradiation with UV light such as black light is put into practical use in security documents, etc. As the toner which emits fluorescence, a toner which emits fluorescence in a visible light region of red, green, or blue color, each of which is one of the three primary colors of light is known. For example, a toner which emits blue fluorescence by irradiation with black light or the like is known.
Such a toner is required to have visibility upon irradiation with UV light such as black light.
In general, commercially available paper contains a fluorescent whitening agent. Paper containing a fluorescent whitening agent emits blue light by irradiation with UV light such as black light. That is, a wavelength region of fluorescence emitted from commercially available paper by irradiation with black light overlaps with a wavelength region of blue fluorescence. Therefore, blue fluorescence emitted from a toner printed on commercially available paper often is hardly distinguished from fluorescence emitted from the commercially available paper. Therefore, the toner which emits blue fluorescence may have insufficient visibility upon irradiation with UV light. Also in a case of a toner which emits green fluorescence, the fluorescence emitted from commercially available paper and the green fluorescence often are hardly distinguished from each other. Therefore, the toner which emits green fluorescence also may have insufficient visibility upon irradiation with UV light.
A toner which emits red fluorescence often has a weak luminescence intensity of the fluorescence, and therefore may have insufficient visibility upon irradiation with UV light. Further, the toner which emits red fluorescence often has low light resistance and low heat resistance, and therefore may not be of practical use.
Examples of a method for improving visibility upon irradiation with UV light include a method in which the content of a fluorescent coloring agent in the toner is increased and a method in which the amount of the toner to be adhered to a recording medium such as paper is increased.
However, when the content of a fluorescent coloring agent in the toner is increased, essential characteristics required for the toner such as dispersibility and low-temperature fixability may be deteriorated. Further, when the amount of the toner to be adhered to paper or the like is increased, even if UV light such as black light is not irradiated, the adhered toner may be easily recognized by the naked eye. Therefore, the above-mentioned methods are not preferred from the viewpoint of security or the like.